There is known a method for ultrasonic inspection of materials (cf. A. K. Gurvich, I. N. Yermolov, "Ultrazvukovoy control svarnych Shvov" /"Ultrasonic Inspection of Welds"/, Technika Publishers, Kiev, 1972, pp. 66-69), which consists in transmitting ultrasonic vibrations into a material, receiving and processing reflected ultrasonic vibrations, separating signals from noise at a present level, and assessing the sizes of flaws.
According to the method under review, the separation of signals from noise at a preset level (cutting off) is effected by limiting signals to a minimum; the present noise cutoff level must be acceptable for practical purposes.
A major disadvantage of the method under review is a distortion of the ratios between signals' amplitudes due to cutting off noise; as a result, it is absolutely impossible to make a correct quantitative evaluation of the sizes of flaws, which is normally done by comparing the amplitudes of signals reflected from a flaw with the amplitude of signals arriving from a reference reflector or the bottom surface of the material being tested.
The method under review is further disadvantageous in that the separation of signals from noise affects the sensitivity of flaw detection because the noise cutoff reduces the absolute values of signals' amplitudes.
There is known a device for ultrasonic inspection of materials, intended for carrying out the foregoing method. In this device, an output of an ultrasonic generator, whose input is connected to one of the outputs of a synchronizer, is connected to an input of an ultrasonic converter which is acoustically coupled to a material being tested; an output of the ultrasonic converter is electrically coupled via a reflected ultrasonic vibrations preprocessing unit and a high-frequency amplifier, which are placed in series, to an input of a unit for separating signals from noise at a preset level, the latter unit's output being connected to one of inputs of a signals recording unit whose other input is connected to a second output of the synchronizer.
In the device under review, the unit for separating signals from noise at a preset level comprises a noise-cutoff detector built around semiconductor diodes; the noise cutoff level is determined by the magnitude of reference voltage set by the operator.
The device has all the disadvantages inherent in the method it is intended to effect: distortion of the initial ratios between signals' amplitudes and lower flaw detection sensitivity due to the noise cutoff. Let it be assumed, for example, that the amplitudes of two signals received at the output of the preprocessing unit are 5 V and 25 V, respectively, and that the level, at which noise is cut off by the detector, is 3 V. Then, the ratio between the amplitudes after cutting off is (5-3): : (25-3)=1:11, the true ratio being 1:5.
Thus, cutting off noises produces different effects upon the amplitudes of signals and changes the initial ratio between these amplitudes, wherefore it is impossible to assess correctly the sizes of flaws.
In order to establish the size of a flaw, one must first discontinue cutting off noise, whereupon the amplitudes of signals are measured and compared against the background of noise.
Naturally, this affects the accuracy of evaluating the sizes of flaws, as well as the sensitivity of the method.
Besides, in the case of automatic inspection, one must stop the inspection in order to measure and compare the amplitudes of signals so as to determine the sizes of flaws; as a result, the rate of inspection is reduced.
Another disadvantage of the device under review resides in the fact that cutting off noise narrows the dynamic range; this, in turn, is due to the fact that after cutting off noise, the gain factor of the receiving channel must be increased so as to ensure effective recording of signals. For example, a noise cutoff, which amounts to 60 percent of the height of the screen of the recording unit's cathode ray tube, halves the dynamic range of the device.
The decrease in absolute values of signals' amplitudes, due to cutting off noise, reduces the reliability of automatic recording of test results.
Besides, the device under review has a low resolution when detecting flaws which are close to one another in both in longitudinal and transverse directions; this equally applies to cases of a low signal-to-noise ratio and may be a serious disadvantage when checking some recently developed alloys which feature a relatively high structural noise level.
There is known another method for ultrasonic inspection of materials (cf. USSR Inventor's Certificate No. 219,639, Cl. GO1 n 29/04), whereby ultrasonic vibrations are transmitted into a material being tested, whereupon the reflected ultrasonic vibrations are received and processed, signals are separated from noise at a preset level, and the sizes of flaws are assessed.
According to the latter method, the separation of signals from noise (the cutoff of noise) is carried out after adding together signals against the background of noise and signals whose level is reduced to a minimum, i.e. after raising the signal-to-noise ratio.
This method, too, does not rule out a change in the initial ratio of signals' amplitude because of cutting off noise, whereby it is impossible to make a correct quantitative evaluation of the sizes of flaws.
True, the reduction in the sensitivity is not as pronounced in the latter method as in the former one, because noise is cut off after the signal-to-noise ratio is improved in advance. However, this method is not effective enough at low signal-to-noise ratios, i.e. when flaws of small sizes have to be dealt with.
The foregoing method is carried out with the use of a device, wherein an output of an ultrasonic generator, whose input is connected to one of outputs of a synchronizer, is connected to an input of an ultrasonic converter which is acoustically coupled to a material being tested. An output of the ultrasonic converter is electrically coupled to one of inputs of a signals recording unit whose other input is connected to a second output of the synchronizer; the electrical coupling is effected via serially placed units which include a reflected ultrasonic vibrations preprocessing unit, a high-frequency amplifier, a no-noise-cutoff detector, and a unit for separating signals from noise at a preset level.
In the above device, the unit for separating signals from noise at a preset level comprises a noise cutoff detector, an adder and a noise cutoff circuit.
An input of the noise cutoff detector is connected to an output of the high-frequency amplifier and an input of the no-noise-cutoff detector; an output of the noise cutoff detector is connected to one of inputs of the adder whose second input is connected to an output of the no-noise-cutoff detector. The adder's output is electrically coupled via the circuit for cutting off noise at a preset level to one of inputs of the signals recording unit. The noise cutoff level is set by the operator and is determined by the magnitude of reference voltage of the noise cutoff detector and the noise cutoff circuit.
The latter device has all the disadvantages typical of the former device. First of all, it alters the initial ratios between the amplitudes of signals and has a low sensitivity when defecting flaws of small dimensions.
Secondly, in the case of automatic inspection the device makes it necessary to stop the inspection in order to evaluate the dimensions of flaws, whereby the rate of inspection is sharply reduced.
Thirdly, the separation of signals from noise at a preset level narrows the dynamic range of the device.
In the fourth place, the device does not provide for reliable automatic recording of signals reflected from small-size flaws when operating at a high noise level.
Finally, the device has a low resolution when inspecting articles manufactured from materials featuring a high level of structural noise.